1. Field of the Invention
The present invention relates to an image forming apparatus for electrophotographic image formation.
2. Description of the Related Art
Conventionally, there has been known an electrophotographic image forming apparatus (e.g., copying machine or printer) for developing a latent image formed on an image carrier (e.g., photosensitive drum) into an image and transferring it onto a recording sheet for image formation. The electrophotographic image forming apparatus includes a developing device in which a latent image is developed using a developer. When a two-component system developer is used for the image development, the developer toner density must be maintained constant to stabilize the resultant image density.
To maintain the developer toner density constant, there has generally been used a method in which the toner density is detected by an optical detection system. As the toner density detection method based on the optical detection system, there are a method based on a developer reflection system and a method based on an image density detection system.
The developer reflection system-based method uses a density detection sensor disposed near a developing sleeve of a developing device on which a two-component system developer with carrier and toner is carried. With the density detection sensor, an amount of reflection light from the developer on the developing sleeve is detected, thereby detecting the developer toner density. On the other hand, the image density detection system-based method forms a reference patch image on an image carrier, and detects an amount of reflection light from the reference patch image to detect the toner density of a developer used for development of the reference patch image.
The image density detection system measures the toner density on the image carrier and controls a toner supply amount within the developing device to make the density of an image actually transferred to a recording sheet constant. However, the amount of reflection light from the reference patch image varies depending also on the manner of bias voltage control for the image carrier and an amount of laser light irradiated onto the image carrier. Therefore, the image density detection system is not often used alone, but often used in combination with a developer reflection system-based density detection sensor for measuring the toner density within the developing device.
Due to a variation (individual difference) in output between developer reflection system-based density detection sensors, there occurs a difference in an amount of reflection light from the developer on the developing sleeve between the sensors, even between those that have the same developer toner density.
To obviate this, there has been proposed a method for reducing a variation in output between density detection sensors (see, for example, Japanese Laid-open Patent Publication No. 2004-110018). This prior art method adjusts an amount of light emission from a density detection sensor so as to make an amount of reflection light from a developer constant in a state that a contamination prevention shutter member is closed, which is disposed to be openable/closable and to face the sensor.
There is another prior art method which controls a density detection sensor such that an amount of light emission therefrom is always made constant. Specifically, at the initial setting of a product (image forming apparatus), reflection light from a developer of predetermined density in a developing device is detected by a density detection sensor, and an output value from the sensor is stored into a memory for use in correcting a variation between sensors.
In the above described prior art techniques, the output of the density detection sensor at a predetermined toner density is controlled to be a constant value. Alternatively, an output value of the density detection sensor at a predetermined toner density is stored in advance, and an output value of the sensor attained when reflection light from a developer currently carried on a developing sleeve is detected is converted into a toner density. However, the output value of the sensor at the predetermined toner density used for the conversion is a mere constant value. In other words, a variation in output without the predetermined toner density between density detection sensors is not taken into consideration.